The human spinal cord is a highly complex system of bones and connective tissues that provide support for the body and protect the delicate spinal column and nerves. The spinal column includes a series of vertebrae stacked one atop the other. Each vertebral body includes a relatively strong cortical bone portion forming the outside surface of the body and a relatively weak cancellous bone portion forming the center of the body. An intervertebral disc is situated between each vertebral body that provides for cushioning and dampening of compressive forces applied to the spinal column. The vertebral canal containing the delicate spinal cord and nerves is located just posterior to the vertebral bodies.
Various types of spinal column disorders include scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine). Other disorders are caused by abnormalities, disease or trauma, such as ruptured or slipped discs, degenerative disc disease (DDD), fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain as well as diminished nerve function.
One known technique to address many such spinal conditions is commonly referred to as spinal fixation. Surgical implants are used for fusing together and/or mechanically immobilizing adjacent vertebrae of the spine. Spinal fixation may also be used to improve the position of the adjacent vertebrae relative to one another so as to alter the overall alignment of the spine. Such techniques have been used effectively to treat the above-described conditions and, in most cases, to relieve pain suffered by the patient. However, as will be set forth in more detail below, there are some disadvantages associated with current fixation devices.
One particular spinal fixation technique includes immobilizing the spine by using orthopedic rods, commonly referred to as spine rods, which run generally parallel to the spine. This is accomplished by exposing the spine posteriorly and fastening bone screws to the pedicles of the appropriate vertebrae. The pedicle screws are generally placed two per vertebra, one at each pedicle on either side of the spinous process, and serve as anchor points for the spine rods. Clamping elements adapted for receiving a spine rod there through are then used to join the spine rods to the screws. The clamping elements are commonly mounted to the head of the pedicle screws. The aligning influence of the rods forces the spine to conform to a more desirable shape. In certain instances, the spine rods may be bent to achieve the desired curvature of the spinal column.
One type of clamping element mounted to a pedicle screw has a housing resembling a saddle connected to the screw. The housing includes a U-shaped channel for receiving a spine rod therein. After the pedicle screw has been inserted into bone and the spine rod is positioned in the U-shaped channel, a set screw is threaded into internal threads of the housing for securing the spine rod in the U-shaped channel.
Surgeons have frequently encountered considerable difficulty when attempting to insert spinal fixation devices. For example, surgeons frequently are unable to efficiently and adequately place the spine rod into the U-shaped heads of the pedicle bone screws because the U-shaped heads of the screws are often not aligned with one another due to curvature in a spine and the different orientations of the pedicle screws. The spine rods are often bent in multiple planes to couple the pedicle screws to the rod, which may lead to weaker connections with the rod. These problems also result in significantly longer operations, thereby increasing the likelihood of complications associated with surgery.
One known solution to some of these problems is a polyaxial pedicle screw that has a spherically shaped head. The spherically shaped head permits movement of the U-shaped housing or rod clamping assemblies relative to the pedicle screw shaft. While the ability to provide a polyaxial orientation for the pedicle screw connection with the spine rod has offered significant benefits, very often such pedicle screw assemblies are complex and difficult for the surgeon to manipulate and install. Such polyaxial pedicle screws often have large and complex rod clamping hardware that is difficult for the surgeon to maneuver around and occupies a large portion of the surgical site.
Moreover, removal or adjustment of the pedicle screw assembly of the spinal fixation system may be required and the complex assemblies associated with known polyaxial rod clamping assemblies make such a procedure very complicated and difficult to achieve, if at all. The ability for the housing of the polyaxial pedicle screw to move relative to the remainder of the spinal fixation system provides significant advantages, but presents many difficulties when removal and/or adjustment of the rod clamping assembly is required, particularly without disrupting the pedicle screw placement in the spine. One source of some of these problems is the deformation of the pedicle screw assembly components or the spinal fixation system components when the spine rod is clamped in place. The components that remain after the spine rod and/or clamping elements is/are removed make reanimation impossible because the components are deformed during initial installation.
Room for improvement of prior art spinal fixation devices remains in the manner of reducing inventories, locking the pedicle screw components, the complexity of use, difficulty in properly positioning the orthopedic rod and the rod-capturing assemblies, the required workspace and manipulation of the many parts associated with some complex devices and the ability to remove specific components without disrupting the remainder of the spinal fixation system.